This invention relates to a focussed ion-beam apparatus which processes a predetermined area on a sample surface by means of a focussed ion-beam which is scanned to irradiate the predetermined area on the sample surface and/or which surveys the sample surface by means of the detection of secondary particles generated from the irradiation by the focused ion-beam. The invention specifically relates to an amplifier circuit connected to a secondary particle detector.
Referring to FIG. 1, the signal treatment of an output signal from the secondary charged particle detector will be explained.
As shown in FIG. 1, the secondary charged particle 14 is detected by the detector 11. Although the range of the output signal X from the detector is determined according to the surface condition of a sample 10, that range does not necessarily coincide with that of an input signal to the image display 13. Then, the calculating operation of the formula (1) is carried out by a first amplifying circuit 12a, in order that a detecting signal X can be optically displayed on the image display. EQU Y=A.times.X+B Formula(1)
The variable Y represents the output of the first amplifying circuit 12a. The variable A represents the contrast coefficient, the variable B represents the brightness coefficient, and both can vary. For example, when the dynamic range of the detector's output signal (maximum/minimum signal) is narrower than that of input signal of the image display 13, as shown in FIG. 2a, the optimization of the signal can be realized by way of the calculating operation of formula (1) through changing the value of the variable A to greater than 1 in the formula (1).
However, depending on the surface condition of the sample, there are situations in which the dynamic range of the detector's output signal becomes broader than that of the image display. In this situation, the amount of information in the detector's output signal can be decreased in the calculating operation by changing the value of the variable A to less than 1 in the formula (1).
In general, the manner in which the value of the term A is determined is by detecting changes in the brightness in the image generated by the secondary particles when the charged particle is scanned to irradiate the sample surface. Included in the observation area are both brighter areas (the area which consists of the material of which the secondary (electron) emission coefficient is bigger, for example the surface exposed metal) and darker area (the area which consists of the material of which the secondary (electron) emission coefficient is smaller, for example silicon oxide film). One shortcoming of this design is that once A and B in the formula (1) are adjusted in order that the brighter area can be observed, the darker area can not be observed, and once it is adjusted so that the darker area can be observed, the brighter area can not be well observed because of halation, where the brighter area brightens too much.
Then, as observed in the formula (2), in order to obtain good image, even when the input signal provides a broad dynamic range, a larger calculating gain A.sub.1 is given to the smaller signals relative to the reference voltage K, and the smaller calculating gain A.sub.2 is given to the bigger signals relative to the reference voltage K. EQU Y.sub.1 =A.sub.1 .times.Y where Y.ltoreq.K Y.sub.1 =A.sub.2 .times.Y where Y&gt;K and where A.sub.1 &gt;A.sub.2 Formula( 2)
In order to obtain A.sub.1 and A.sub.2 in the formula (2), in prior art as shown in FIG. 3, the second amplifying circuit 12b (In-phase amplifying circuit) is arranged in series behind the first amplifying circuit 12a, where R.sub.1 =R.sub.4, and R.sub.2 =R.sub.3.
When the input signal Y is smaller than the threshold voltage VD in forward direction of diode 26 (the reference voltage K), the diode 26 is set to an open condition and the output signal Y.sub.1 is given as in the formula (3) EQU Y.sub.1 =(R.sub.1 /R.sub.2 +1)Y Formula(3)
That is, R.sub.4 /R.sub.3 +1 is identical to A.sub.1 in the formula (2).
And, when the input signal Y is bigger than VD, the diode is set to a short-circuit condition, and the output signal Y.sub.1 is given as in the formula (4). EQU Y.sub.1 =Y+(R.sub.1 /R.sub.2)VD Formula(4)
In this case, 1 is identically equal to A.sub.2 in the formula (2).
Thus, the image is displayed, in prior art, in that higher gain is given to the lower signal and that lower gain is given to the higher signal.
The traditional circuitry is mainly used as for the signal processor of the scanning type electron microscope. In the case of the electron microscope, the electron is the preliminary charged particle, and the maximum secondary electron emission coefficient in the brighter area may be only 10 times greater than the secondary electron emission coefficient in the darker area. Therefore, the second term in the formula (4) does not become so big since the operating gain (R.sub.1 /R.sub.2 +1) in the smaller signal inputs is relatively small. As a result, a problem does not occur.
But in the focussed ion-beam apparatus, the maximum secondary charged emission coefficient in the brighter area can be 100 times greater than the maximum secondary charged emission coefficient in the darker area, since the primary charged particle is an ion. Therefore, when a sufficiently large operating gain (R.sub.1 /R.sub.2 +1) is given to the smaller signals, a problem occurs that a good image can not be obtained because the second term (VDR.sub.1 /R.sub.2) in the formula (4), which is the error term, becomes so big that it can not be treated as negligible.